1. Field of the Invention
The present invention relates generally to the treatment of a re-pulped coated broke slurry with a water-soluble dispersion polymer so that it can be recycled as cellulose fiber to a paper machine.
2. Description of the Prior Art
"Paper Broke" is a term used by papermakers to describe that paper which they cannot or do not sell because it does not meet minimum commercial specifications. This paper broke is a valuable source of fiber and is recycled internally at the mill although it may also be sold to other mills as a source of fiber. Unfortunately, paper broke frequently contains coatings that are applied to the base sheet of paper as it is being manufactured. When the paper broke contains these coatings it is referred to as a "Coated Broke". Coated broke presents special problems in the recovery of fiber values because the coatings introduce materials which would not normally be present in the original stock of fiber used to manufacture the base paper sheet.
The coating materials contained on coated broke may account for from about ten (10) to about forty (40) weight percent of the total solids in the paper finish. The major components of the coatings are pigments which normally constitute from about 80 to 95% of the coating mass, and the binders are contained on the coating from about 5 to about 20 weight percent of the coating mass.
The pigments normally are composed of typical pigments and fillers used in manufacture of paper, which pigments and fillers can include clays of various types, calcium carbonate, titanium dioxide, and other similar or specialty pigments and fillers.
The binders used are frequently those binders obtained from normal latex polymers such as those derived from styrene-butadiene resins, polyvinyl acetate resins, polyvinyl alcohol resins, and polyacrylic or polyacrylate resins. Certain binders can be customized depending upon the end result desired by the papermaker.
The combination of these binder materials, which can also include certain natural products such as starches and dextrans, with the pigments and fillers earlier mentioned, all of which are contained as part of the coating in a coated broke presents certain problems when the coated broke is recycled to recover fiber values.
The most difficult problem involved with recycling of coated brokes is derived from the binder materials, sometimes in combination with pigments or fillers, since these polymers and the materials to which they have been attached, are the origin of sticky deposits. These sticky deposits, referred to as "white pitch" cause difficulties when recycled back to the paper machine operation. In addition to these white pitch sticky deposits, problems that are caused can include, but are not necessarily limited to, those problems associated often with the standard pitch derived from, natural wood fibers. The problems caused by inclusion of this white pitch in the papermaking process using recycled coated brokes can include off specification paper caused by holes and/or deposits of the white pitch, machine down time resulting from sheet breaks or more frequent machine cleanup, clogging of the felts used in the manufacture of the base sheet, and the like.
In the past, polymers derived from crosslinked or linear epichlorohydrin/dimethylamine (EPI-DMA) reactants have been used to treat coated broke. These materials, though effective in certain coated broke applications, have difficulties of their own primarily derived from the fact that the materials may be crosslinked and can form gel particles which provide their own difficulties in further processing of the paper sheet. In addition, although this EPI-DMA material is highly cationically charged, as originally considered necessary for this type of application, it has been found that this very high cationic charge density is not necessary for effective treatment of coated broke and the white pitch derived therefrom.
Recently, copolymers containing the monomer diallyl dimethyl ammonium chloride (DADMAC) and acrylamide have been added to re-pulped coated broke slurries for the purpose of coagulating white pitch. Such a method is disclosed in U.S. Pat. No. 5,131,982, issued to St. John, the disclosure of which is herein incorporated by reference. Homopolymers of DADMAC have also been suggested for use in the St. John patent.
These coagulants come as either solution polymers or as water-in-oil emulsion polymers. Solution polymers are limited to lower molecular weight polymers at relatively low concentrations. It is known that higher molecular weight polymers can provide improved treatment of coated broke. One way of obtaining higher molecular weight polymers in a liquid form is to package the polymer in a water-in-oil emulsion. This type of polymer, though, typically requires more elaborate feeding equipment than that required for the solution polymers, and this has caused a great reluctance amongst papermakers to using this type of polymer.
Certain hydrophobic dispersion polymers have been suggested as a means of overcoming the above problems. U.S. Pat. No. 5,466,338, issued to Pearson, the disclosure of which is incorporated herein by reference, discloses a method of treating coated broke by utilizing copolymers and terpolymers of benzyl chloride quaternaries. In these polymers the hydrophobic moiety prevents the polymer from fully solubilizing and reaching full activity.